General Descriptions of Aluminum Electolytic Capacitors
1. General Description of Aluminum Electrolytic Capacitors
1-1 Principles of Aluminum Electrolytic
Capacitors
1-2 Capacitance of Aluminum Electrolytic
Capacitors
An aluminum electrolytic capacitor consists of cathode
aluminum foil, capacitor paper (electrolytic paper),
electrolyte, and an aluminum oxide layer, which acts as
the dielectric, formed on the anode foil surface.
A very thin oxide layer formed by electrolytic oxidation
(formation) offers superior dielectric constant and has
rectifying properties. When in contact with electrolyte, the
oxide layer possesses an excellent forward direction
insulation property. Together with magnified effective
surface area attained by etching the foil, a high
capacitance yet small sized capacitor becomes available.
As previously mentioned, an aluminum electrolytic
capacitor is constructed by using two strips of aluminum
foil (anode and cathode) with paper interleaved. This foil
and paper are then wound into an element and
impregnated with electrolyte. The construction of an
aluminum electrolytic capacitor is illustrated in Fig. 1-1.
The capacitance of an aluminum electrolytic capacitor
may be calculated from the following formula same as for
a parallel-plate capacitor.
-- 8 εS
(1 - 1)
(µF)
C = 8.855 10
d
,
,,,,,,,
,,,,,,
,
,,,,,,,
Electrolyte
(Real Cathode)
Anode aluminum
electrode
Oxide film
Cathode Aluminum
electrode
(Apparent Cathode)
Electrolytic paper
containing electrolyte
Fig. 1 - 1
Since the oxide layer has rectifying properties, a
capacitor has polarity. If both the anode and cathode foils
have an oxide layer, the capacitors would be bipolar (nonpola) type capacitor.
These technical notes refer to "non-solid" aluminum
electrolytic construction in which the electrolytic paper is
impregnated with liquid electrolyte. There is another type
of aluminum electrolytic capacitor, which is the "solid" that
uses solid electrolyte.
ε : Dielectric constant of dielectric
S : Surface area (cm2) of dielectric
d : Thickness (cm) of dielectric
To attain higher capacitance "C", the dielectric constant
" ε " and the surface area "S" must be large while the
thickness "d" must be small. Table 1-1 shows the
dielectric constants and minimum thickness of dielectrics
used in various types of capacitors.
With aluminum electrolytic capacitors, since aluminum
oxide dielectric has excellent withstand voltage per
thickness. And the thickness of dielectric can be freely
controlled according to the rated voltage of the aluminum
electrolytic capacitor.
Therefore, in compare to other dielectric, similar voltage
endurance is provided by aluminum oxide dielectric even
if thickness ("d" in the above formula) is thin.
Furthermore, by etching the surface of aluminum foil,
the effective area of the foil as compared to the apparent
area can be enlarged 80~100 times for low voltage
capacitors and 30~40 times for middle / high voltage
capacitors. Therefore, aluminum electrolytic capacitors
have a higher capacitance for a specified apparent area
than other types of capacitors.
High purity aluminum foil for the anode is etched by
electrochemical process in a chloride solution with DC,
AC, or an alteration of DC and AC, or a concurring AC and
DC current.
Fine surface etching (photo 1-1)
accomplished mainly by AC electrolysis is generally used
for low voltage foil. Tunnel etching (photo 1-2)
accomplished mainly by DC electrolysis is used for middle
/ high voltage foil. The ething of the cathode foil is mainly
accomplished by AC electrolysis to increase the surface
area.
Table 1-1 Dielectric constants and minimum thickness of dielectrics used in various types of capacitors
Type of Capacitor
Dielectric
Aluminum Electrolytic Capacitor
Aluminum Oxide
Tantalum Electrolytic Capacitor
Film Capacitor (Metallized)
Dielectric Constant
ε
Dielectric Thickness d (µm)
7~10
(0.0013~0.0015/ V)
Tantalum Oxide
24
(0.001~0.0015/ V)
Polyester Film
3.2
0.5~2
Ceramic Capacitor (High Dielectric Constant Type)
Barium Titanate
500~20,000
5
Ceramic Capacitor (Temp. Compensation Type)
Titanium Oxide
15~250
5
TECHNICAL NOTES CAT.8101C
NICHICON CORPORATION
General Descriptions of Aluminum Electolytic Capacitors
Section
Surface
Photo 1-1 Surface and section photo of etched aluminum foil for low voltage capacitors.
Section (Replica)
Surface
Photo 1-2 Surface and section photo of etched aluminum foil for middle / high voltage capacitors.
TECHNICAL NOTES CAT.8101C
NICHICON CORPORATION
General Descriptions of Aluminum Electolytic Capacitors
1-3 Dielectric (Aluminum Oxide Layer)
0.0013~0.0015 (µm/ V).
Expanded photography of a dielectric (aluminum oxide
layer) on the foil that has not been etched (plain foil) is
shown in photo 1-3.
The fabrication reaction of the dielectric can be
expressed as follows:
[Al(OH2)6]3+
1) Al3++6H2O
3+
Al(OH)3+3H2O+3H+
2) [Al(OH2)6]
Al2O3+3H2O
3) 2Al(OH)3
20V Fomed
Photo 1-3
dielectric
dielectric
dielectric
A high purity etched aluminum foil is anodized in a
boric acid-ammonium water type solution, for exsample,
to form an aluminum oxide layer on its surface. This
aluminum oxide layer is what we call the dielectric of the
aluminum electrolytic capacitor. The DC voltage that is
applied to the foil to oxidize the anode foil is called
"Forming Voltage".
The thickness of the dielectric is nearly proportional to
the forming voltage and measures approximately
100V Fomed
250V Fomed
Enlarged photo of oxide layer formed on a non-etched plain aluminum foil.
Photo 1-4
Enlarged photo of middle, high voltage formed foil.
(Condition of oxide layer formation in a pit)
1-4 Electrolyte
Anode foil and a cathode foil facing each other are
interleaved with electrolytic paper and wound into a
cylindrical shape. This is called a "capacitor element." At
this stage, it has configuration of a capacitor when
considers electrolytic paper and the aluminum oxide layer
to be dielectric, however, the unit has few capacitance.
When this capacitor element is impregnated with liquid
electrolyte, the anode foil and cathode foil are electrically
connected. With the aluminum oxide layer formed on the
anode foil acting as the sole dielectric, a capacitor with a
high value of capacitance is now attainable. That is to say
that the electrolyte is now functioning as a cathode. The
basic characteristics required of an electrolyte are listed
below:
(1) It must be electrically conductive
(2) It must have a forming property to heal any flaws on
the dielectric oxide of the anode foil.
(3) It must be chemically stable with the anode and
cathode foils, sealing materials, etc.
(4) It must have superior impregnation characteristics
(5) Its vapor pressure must be low.
The above characteristics of electrolyte greatly
influence the various characteristics of aluminum
electrolytic capacitors. For this reason, the proper
electrolyte is determined by the electrical ratings,
operating temperatures and the application of the
capacitor.
NICHICON CORPORATION
TECHNICAL NOTES CAT.8101C
General Descriptions of Aluminum Electolytic Capacitors
1-5 Manufacturing Process of Aluminum
Electrolytic Capacitors
Process
Etching
(Enlargement of the
Surface Area)
Main Materials
Contents
High purity aluminum foil
A 0.05~0.11 mm thick anode foil and a 0.02~0.05 mm thick
cathode foil are continuously etched electrochemically in a
chloride solution with an AC or DC current. This enlarges the
effective surface area of the aluminum foils to attain smaller
capacitor sizes.
Chloride
Pure water
Forming
Etched foil
(Formation of
the Dielectric)
Borate, etc.
Slitting
The process develops aluminum oxide (Al203) to form a
capacitor dielectric.
A high purity etched aluminum foil is anodized in a boric acidammonium water type solution, for exsample, to form an
aluminum oxide film on the surface of the cathode foil. As for
the anode foil, an low DC voltage is sometimes used for
formation, but there are also situations where formation is not
conducted.
Pure water
A foli is slit into specified widths according to capacitor case
sizes.
Anode foil
Cathode foil
Slited foils (anode/cathode)
Winding
Electrolytic paper
Leade
Anode and cathode foils interleaved with an electrolytic paper
are wound into a cylindrical capacitor element, with leads
being connected to both foils.
Winding aftixing material
Lead
Winding aftixing
material
Element
Foil
Electrolytic
paper
Fig. 1 - 2
TECHNICAL NOTES CAT.8101C
NICHICON CORPORATION
General Descriptions of Aluminum Electolytic Capacitors
Prosess
Main Materials
Element
Impregnation
Electrolyte
Contents
Elements are impregnated with liquid electrolyte.
The clearance between the two electrode foils is filled with
liquid electrolyte. With this impregnation, an element can
function as a capacitor.
The impregnated element, case and end seal are assmbled.
Elements
Electrolyte
For the end seal , a rubber packing, a rubber lined bakelite
(with terminals) or a molded plastic plate (with terminals) are
Case (usually aluminum)
used.
End seal
After assembly, the capacitors are covered with exterior
Rubber packing,
housing material. Sleeving is not used for laminate case
Rubber-bakelite with terminals products, such as surface mount capacitors.
Impregnated elements
Assembly
&
Finishing
(
Mold plastic with terminals
Exterior housing material
(Sleeving, bottom plate, etc.)
)
Aluminum rivet
Lead wire
Terminal
Rubber-bakelite
Aluminum lead tab
Curled section
,,,,,,
,,,,
,,,
Aluminum lead tab
Rubber packing
,,, ,
,,,,
,,,,
, ,,
,
Curled section
Aluminum case
Sleeving
Aluminum case
Element
Sleeving
Element
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,
Fig. 1 - 3
Fig. 1 - 4
Element fixing material
(may or may not
be - used)
Bottom plate
Nylon laminate casing for laminate products (sleeveless)
Aging
Processing
Assembled products
DC voltage is applied under high temperature conditions to
reform the oxide film.
Plastic platform
Leads are processed and the plastic platform is attached to
surface mount capacitors.
Depending on customer specification, the lead cutting,
forming, snap - in and taping are processed.
Accessories, such as mounting braket, are attached.
Taping material
,,,,
,,,,
,,,,
,,,
Accessories
(Capacitor mounting braket
terminal screws , etc.)
Fig. 1 - 5
An inspection based on the standard specification and test
requirements is performed to guarantee the quality of
products.
Inspection
Packaging
Packaging materials
Shipping
TECHNICAL NOTES CAT.8101C
NICHICON CORPORATION
General Descriptions of Aluminum Electolytic Capacitors
1-6 Characteristics
1-6-1 Capacitance
The capacitance of the dielectric portion of the anode
aluminum foil can be calculated with the following formula
(discussed in 1-1) :
Z = R2+(ω L -- ω C )
ε S (µF)
-- 8
Ca = 8.855 10
1
d
The cathode foil has a capacitance (Cc) that uses the
oxide film, which formed by the forming voltage or formed
naturally during storage (generally it should be 1V or less),
as a dielectric. According to the construction of aluminum
electrolytic capacitors, Ca and Cc are connected in a
series. Therefore, the capacitance can be determined by
the following formula:
Ca C c
Ca + Cc
10
The standard capacitance tolerance is 20%(M);
however, capacitors with a capacitance tolerance of
10%(K), etc. are also manufactured for special usage.
The capacitance of aluminum electrolytic capacitors
changes with temperature and frequency of measurement,
so the standard has been set to a frequency of 120Hz and
o
temperature of 20 C.
10-1
R
10-2
10-3
XL
Xc
102
103
104
105
106
Frequency (Hz)
Fig. 1 - 8
C Capacitance (F)
r
R
r Equivalent parallel resistance of
anode oxide film (Ω)
R Equivalent series resistance (Ω)
L
C
Fig. 1 - 6
L Equivalent series inductance ( H )
A reactance value due to the equivalent series
inductance "L" is extremely small at low frequencies
(50Hz~1kHz) and can be regarded as zero. Therefore,
the following formula can be set up.
tanδ =
Z
R
=ωCR
Xc
DF = tan δ
δ
(1/ωc)
1
Z
1-6-2 Equivalent Series Resistance (R), Dissipation
Facter (tanδ), Impedance (Z)
The equivalent circuit of an aluminum electrolytic
capacitor is shown below, The equivalent series resistance
is also known as "ESR".
Xc
2
Its relation with frequencies is shown by a model curve.
The inductance "L" is mainly from the wound electrode
foils and the leads.
ESR "R" is from resistance of the electrode foils, the
electrolyte, the leads and each connection.
R (Ω), Z(Ω)
C=
The impedance can be expressed by :
1
Z=
+ jw L + R
jω C
Its absolute value can be expressed by :
PF = cos θ
θ
R
Q =
1
tan δ
(ω = 2 π f)
Fig. 1 - 7
(1 - 2)
(1 - 3)
100 (%)
R
=
=
Z
Xc
= R
R
2
R +
2
(1 - 4)
( ω1C )
(1 - 5)
1-6-3 Leakage Current
The causes of leakage current in aluminum electrolytic
capacitors are listed below :
1)Distorted polarization of dielectric (aluminum oxide
layer)
2)Resolution and formation of dielectric
3)Moisture absorption by dielectric
4)Breakdown of dielectric due to the existence of
chlorine or iron particles.
The leakage current value can be decreased by proper
selection of materials and production methods; however,
cannot be totally eliminated.
Leakage current is also dependent upon time, applied
voltage and temperature.
The specified leakage current value is measured after
the rated voltage of the capacitor is applied at room
temperature for a specified time period. When selecting a
capacitor for a particular application, characteristics such
as temperature dependency, aging stability and etc. must
be taken into account.
TECHNICAL NOTES CAT.8101C
NICHICON CORPORATION
General Descriptions of Aluminum Electolytic Capacitors
1-6-4 Temperature Characteristics
Aluminum electrolytic capacitors have liquid electrolyte.
This electrolyte has properties (conductivity, viscosity,
etc.) that have rather conspicuous temperature
characteristics.
Electrical conductivity increases as the temperature
increases and reduces as the temperature decreases.
Therefore, the electrical characteristics of aluminum
electrolytics are affected by temperature more than other
types of capacitors. The following section explains the
relationship between temperature and capacitance,
tangent delta, ESR, impedance and leakage current.
10
-25oC
1
tan δ
0.01
100
+20oC
tan δ
+65oC
+105oC
0.1
50V 1000µF 105o C
0.01
100
10k
1k
Frequency (Hz)
Fig. 1 - 10 Tanδ vs. Frequency
Characteristics
at 120HZ
-10
-20
10
-30
50V 1000µF 105oC
-40
20
10
-40
-20
0
20
40
60
Temperature (o C)
80
100
120
ESR( Ω )
Impedance( Ω )
-60
Capacitance change (%)
-25oC
1
200V 470µF 105 o C
0
-10
1
at 120HZ
0.01
100
-30
10
-40
-40
ESR
+20oC
-20
-60
Impedance
-25oC
0.1
-20
0
20
40
60
Temperature (o C)
80
100
120
Fig. 1 - 9 Capacitance vs. Temperature Characteristics
1k
10k
Frequency (Hz)
+65oC
+105oC
100k
200V 470µF 105oC
-25oC
ESR( Ω )
Impedance( Ω )
Capacitance change (%)
0
10k
1k
Frequency (Hz)
10 200V 470µF 105oC
2) Tanδ, Equivalent Series Resistance (ESR), Impedance
The Tanδ, equivalent series resistance (ESR) and
impedance changes with temperature and frequency. An
example of the general characteristics is shown in Fig. 110 and 1-11.
10
+20oC
+65oC
+105oC
0.1
1) Capacitance
The capacitance of aluminum electrolytic capacitors
increases as the temperature increases and decreases as
the temperature decreases. The relationship between
temperature and capacitance is shown in Fig. 1-9.
20
50V 1000µF 105oC
1
Impedance
ESR
0.1
+20oC
+65oC
0.01
100
+105oC
1k
10k
Frequency (Hz)
100k
Fig. 1 - 11 Impedance, ESR vs. Frequency
Characteristics
TECHNICAL NOTES CAT.8101C
NICHICON CORPORATION
General Descriptions of Aluminum Electolytic Capacitors
4) Leakage Current
The leakage current increases as the temperature
increases and decreases as the temperature decreases.
Fig. 1-12 shows the relationship between temperature and
leakage current.
1000
Leakage current (µA)
3) Impedance Ratio
o
The ratio between the impedance at 20 C and the
impedance at various temperatures is called the
impedance ratio. Impedance ratio becomes smaller as
smaller change of ESR and capacitance with temperature.
The quality of performance at low temperatures is
particularly expressed with the impedance ratio at 120Hz.
50V 1000 µF 105oC
Measured after 1 minute
application of rated voltage
100
10
1
-20
Leakage current (µA)
1000
0
20
40
60
Temperature (oC)
80
100
120
80
100
120
200V 470 µF 105oC
Measured after 5 minutes
application of rated voltage
100
10
1
-20
0
20
40
60
Temperature (oC)
Fig. 1 - 12 Leakage current vs. Temperature Characteristic
TECHNICAL NOTES CAT.8101C
NICHICON CORPORATION